This circuit for testing old analog lines was published in April 1986 Radio Electronics magazine. The circuit uses only passive components.
This circuit that triggers an SCR was found in volume 5 of the 1995 Encyclopedia of Electronic Circuits....
This 1-transistor oscillator circuit was found in the March 1965 Radio Electronics magazine. The...
Found in a 1964 CQ magazine this circuit consists of an automatic telegraphic key, providing the...